Race and genetics form their own double helix, twisting
together through history. The Nazis, as everyone knows,
justified the death camps on the grounds that Jews and
Gypsies were genetically inferiorbut what is less known is
that the Nazis took their cue from eugenics legislation
passed in the United States. Here, race is defined primarily
by skin color. Since that's a genetic trait, the logic goes,
race itself must be genetic, and there must be differences
that are more than skin deep.

But that's not what modern genetics reveals. Quite the
contrary, it shows that race is truly skin deep. Indeed,
genetics undermines the whole concept that humanity is
composed of ''races''pure and static groups that are
significantly different from one another. Genetics has
proven otherwise by tracing human ancestry, as it is
inscribed on DNA.

Demystifying race may be the most important accomplishment
of this research, but it has also solved some of the most
intriguing mysteries of human history.

In 1918 a wounded woman showed up in a Berlin mental
hospital claiming to be Anastasia, the last surviving member
of the Russian imperial Romanoff family. Her story, from
which she never wavered, engendered an epic controversy that
ranged from courtrooms to the silver screen. The mysterious
woman married an American, took the name Anna Anderson, and
died in 1984, insisting to her grave that she was the true
Anastasia.

After her death, an amateur historian bought some of
Anderson's books.

In one was an envelope with some strands of her hair. He
took them to Mark Stoneking, a Penn State University genetic
anthropologist who would later confirm the identity of Jesse
James's remains. Meanwhile an English geneticist had
obtained some of Anderson's colon tissue that a hospital had
stored after an operation. Both researchers analyzed the
DNA. ''We found that our sequences matched each other,''
Stoneking recalls, ''but they didn't match the royal
family.''

So who was Anna Anderson? ''One of the private investigators
hired by other Russian nobility came to the conclusion that
she was a Polish woman who had been working in a munitions
factory,'' Stoneking says. There had been an explosion at
this factory, which could explain the wounds that gave such
credence to her tale of fleeing the Bolsheviks. The English
team tracked down a relative of this Polish woman, and,
indeed, her DNA matched Anna Anderson's.

If the Polish relative had come from the paternal side of
Anderson's family, the English team would have been at a
dead end. That's because they were analyzing something
called mitochondrial DNA. Almost all human cells contain
tiny bacteria-like entities called mitochondria. They
provide energy to cells, and they have their own DNA,
separate from the DNA that actually makes a person.
Mitochondria are not in sperm cells; therefore, they are
inherited only from the mother. They record a person's
matrilineal heritage.

The paternal counterpart is the Y chromosome. Women, of
course, lack the Y chromosome, so it is inherited strictly
from father to son. It can be quite revealing to trace how
the Y chromosome and mitochondrial DNA mix in a single
population. Under the old South African apartheid
categories, ''colored'' people were those who descended from
black and white parentsbut their Y chromosome almost
always shows a European ancestry, whereas their
mitochondrial DNA usually shows an African heritage. To put
it plainly, white men were sleeping with black women, but
black men were not sleeping with white women.

This pattern is common wherever one finds ''dominant and
subservient groups,'' says South African researcher Himla
Soodyall. In southern Colorado, for example, a group of
Hispanics trace their ancestry to Spanish settlers from the
1500s, before Jamestown. ''Their oral history says they
didn't mix with the native Americans,'' says University of
Michigan researcher Andrew Merriwether, who studied this
group. But genetics tells a different tale: about 85 per
cent of them carry mitochondrial DNA of Native American
origin. Other genetic markers show a strong European
heritage, which indicates ''directional mating,'' says
Merri- wether. As in South Africa, European men were
sleeping with Amerindian women, but Amerindian men were
rarely sleeping with European women. Partly this is because
few Spanish women traveled with the conquistadores, but it's
also due to sexual politics, and they are inscribed on DNA.

So are ancient human migrations. Thor Heyerdahl believed
that Polynesians crossed the Pacific and helped populate the
New World. By sailing his boat, the Kon Tiki, he proved such
a voyage was possiblebut DNA demonstrates that it didn't
happen. Polynesians bear a distinctive motif on their
mitochondrial DNA that is not present among any native
American peoples, either those who are living now or
mummies. So did the first Americans come from Siberia?
Surprisingly, no. Mitochondrial DNA indicates that native
Americans descend from Mongolians.

Such genetic history depends on statistics. Researchers test
hundreds or thousands of people in a given population to
find what motifs are present and in what concentrations.
Then they look for other populations that possess the same
markers. ''We try to construct the most likely historical
scenario,'' explains Stoneking, ''but we can't rule out more
complicated alternatives.'' He says scientists must
triangulate ''the fossil, archaeological, and genetic
evidence.''

But sometimes only DNA can settle questions of human
history. Europeans almost all descend from farmers who
slowly moved northeast from what is now Turkey. They
subsumed the hunter-gatherers whom they encountered, but
pockets of the old hunters still remain. The Saami
peopleformerly known as the Lappslive in Scandinavia and
speak a language close to Finnish. Finns and Saamis ''used
to say they had a common history, one that goes back to
Romantic myths of coming from the Urals,'' says University
of Munich researcher Svante Paabo. Genetically, the Saami
are indeed distinct from the mass of Europeans. ''But the
Finns look like everyone else in Europe,'' says Paabo. ''The
Finns borrowed their language from the Saami, probably when
they came as farmers. Then they pushed away the Saami by
taking more and more land.'' The Basques also seem to be an
outpost of the earlier hunters; their DNA carries different
motifs than that of the surrounding Europeans.

Japan was populated by ancient Koreans and, earlier, by a
mysterious people called the Jomon, known only by their
pottery and other archaeological remains. Where did they
come from? To figure that out, geneticist Michael Hammer of
the University of Arizona looked at the Y chromosome.
Surprisingly, the closest match to the Jomon variant lies in
Tibet. How could an isolated mountain tribe thousands of
miles from the sea be related to the first Japanese? The
Tibetans and the Jomon might descend from a common tribe
that lived in central Asia, where the Jomon-Tibetan motif is
now found only rarely, superseded, perhaps, by the ceaseless
mixing of people. But it might also be that migrants from
Tibet crossed Asia and entered Japan on an ice bridge 12,000
to 22,000 years ago.

Even individuals can sometimes trace their heritage. (See
box, Roots, DNA Style.) Matthew George, a geneticist at
Howard University, is analyzing the DNA from bones found in
the African Burial Ground in Wall Street's Foley Square.
Since lab contamination is always a danger, he says, ''we
test our own mitochondrial DNA.'' He recalls that an African
American colleague had DNA that was closely related to
people in Benin. ''She started dancing around saying, 'Oh,
I'm from Benin, I'm from Benin.' I said, 'No, you're from
Plains, Georgia. But, yes, your mitochondrial DNA comes from
Benin.'''

With the promise of genealogy comes the danger of bigotry.
Genetic classification could ''concretize the racist
assumptions already out there in the scientific milieu,''
warns University of Maryland anthropology and biology
professor Fatimah Jackson. ''This isn't an idle fear I
have.''

Others share her uneasiness. Ashkenazi Jews are much more
likely than other groups to have a mutation that causes
breast and ovarian cancer. New York magazine recently called
this the ''Jewish gene,'' even though non-Jews can also
carry it. In the shadow of the Holocaust, some Jews worry
about being stigmatized as genetically inferior.

So do African Americans. ''Medical literature is replete
with black-white distinctions,'' says Jackson, and many of
them are based on bad science. ''You realize they sampled 12
black men in Chicago, who are supposed to stand for all
African Americans. Science begins with the collection of the
sample and the definition of the group to be studied.''

The impact of what Jackson calls ''lazy genetics'' can be
devastating. ''With anemia,'' she recalls, ''physicians were
being told, 'When you see low hemoglobin levels in a black
child, that's not anemia, it's just genetic and you don't
need to treat. But the same level in a white child needs
treatment.' So they disenfranchised all these people by
geneticizing what might have been environmental.''

Specific problems such as this arise from a general set of
assumptions about race. Biology textbooks used to show the
ascent of man, leading from apes through Africans and Asians
and culminating with Europeans. These racist hierarchies
were justified in part by evolutionary theory. Two million
years ago, various hominid ancestors of modern humans
migrated out of Africa. Neanderthals settled in Europe--and
some scientists argued that Europeans descend from
Neanderthals, Asians from other hominids such as Peking Man
or Java Man, and Africans from still other sources. Genetics
has helped demolish this ''multiregional'' theory.

Mitochondrial DNA indicates that all living humans descend
from one maternal sourcechristened Mitochondrial Evewho
lived in Africa between 100,000 and 200,000 years ago.
Similarly, the Y chromosome shows that all men have a common
ancestor, Y-chromosome Adam, who lived at the same time.
(Actually, both analyses indicate that modern humans descend
from a small founding population of about 5000 men and an
equal number of women.) The time estimates are based on
assumptions on how frequently genetic mutations occur. The
mutation clocks of mitochondrial DNA and the Y chromosome
tick at different speeds, so the fact that they both
indicate humans emerged at the same historical moment makes
this evidence much more convincing.

Did modern humans coming out of Africa completely replace
Neanderthals and the other earlier hominidsor did they
interbreed with them? This year, Stoneking and researchers
in Germany compared the mitochondrial DNA of modern humans
to that of a Neanderthal skeleton between 30,000 and 100,000
years old. The conclusion: Neanderthals contributed nothing
to human maternal ancestry.

But, says Svante Paabo, who led the Neanderthal project, the
question of whether humans mated with other hominids, such
as those in Asia, is still open. ''The ultimate answer will
be to look at 100 or more loci in the genome,'' he says.
''If it all comes from Africa, then that would prove''
humans from Africa colonized the globe, replacing their
older hominid cousins. But, he says, ''I find it hard to
believe that there would have been absolutely no
interbreeding, that it would be such a simple story.''

Indeed, the Y chromosome has begun to tell a more
complicated tale. ''We found that the oldest branches in the
Y chromosome tree trace to Africa,'' explains Hammer. ''But
an intermediate-length branch seems to originate in Asia,
and that one led to a newer branch in Africa.'' In fact,
says Hammer, ''the majority of the Y chromosomes in Africa
seem to be derived from one that may have come from an Asian
source.'' Hammer thinks that after the initial human
diaspora out of Africa, there was a reverse migration back
into Africa between 10,000 and 50,000 years ago.

This doesn't prove Homo Sapiens bred with other hominids:
Hammer's Asian Y chromosome could have arisen by mutation,
not by interbreeding. But if some breeding with older
hominids is proven, might that rekindle the old racist
genealogies? Hammer doesn't think so. ''Each trait is
floating around out there in geographical space,'' he says.
In other words, every person's DNA is a mosaic of segments
that originated at various times and in different places.

That helps explain a fundamental finding: Genetic variation
within any race is much greater than between races. ''If you
take even a small camp of Pygmies,'' says L. Luca
Cavalli-Sforza, a pioneer of genetic anthropology, ''they
are extremely different for all the genetic markers we look
at.'' Indeed, they show almost all the genetic variation
catalogued in the world.

Racial hierarchies are cultural, not scientific. While every
group has genetic characteristicsand sometimes flawsthat
are more common than in other groups, not everyone in the
group will share them. The Afrikaners, much more than South
Africa's other ethnic groups, are prone to porphyria
variegata, the blood disorder depicted in the film The
Madness of King George. It turns the urine purple and can
incite temporary insanity. Almost all the South African
cases of this disease can be traced to a single Dutch couple
who married in Capetown in 1688. Being an Afrikaner is not a
risk factor; being a descendant of this couple is.

Not only is race or ethnicity a poor predictor of most
genetic traits, it is very hard to define. Many people think
they can easily tell an Asian from a European, but, says
Paabo, ''If we start walking east from Europe, when do we
start saying people are Asian? Or if we walk up the Nile
Valley, when do we say people are African? There are no
sharp distinctions.''

Cavalli-Sforza has probably spent more time trying to
classify human groups by genetic analysis than anyone else.
In his massive book The History and Geography of Human
Genes, he groups people into geographic and evolutionary
clusters--but, he writes, ''At no level can clusters be
identified with races.'' Indeed, ''minor changes in the
genes or methods used shift some populations from one
cluster to the other.''

Geneticist Steve Jones makes this point by looking at blood.
''We would have a very different view of human race if we
diagnosed it from blood groups, with an unlikely alliance
between the Armenians and the Nigerians, who could jointly
despise the...people of Australia and Peru,'' who generally
lack type-B blood, Jones writes in The Language of Genes.
''When gene geography is used to look at overall patterns of
variation,'' he writes, ''color does not say much about what
lies under the skin.''

Not only is our concept of race arbitrary, but it is based
on a relatively insignificant difference between people.
Skin pigment, eye shape, and hair type are all determined by
genes. Indeed, as the human genome is mapped, geneticists
might be able to reconstruct what mummies or other ancient
people looked like. But the physical ''stereotypes'' of
race, writes Cavalli-Sforza, ''reflect superficial
differences.'' For example, light skin color is needed in
northern climates for the sun's ultra- violet light to
penetrate into the body and transform vitamin D into a
usable form. This mutation may well have arisen at different
times, in different ancestral groups, on different points
along the DNA. That's true for cystic fibrosis, which occurs
almost exclusively in people of European descent but is
caused by several different mutations.

In other words, ''white people'' do not share a common
genetic heritage; instead, they come from different lineages
that migrated from Africa and Asia. Such mixing is true for
every race. ''All living humans go back to one common
ancestor in Africa,'' explains Paabo. ''But if you look at
any history subsequent to that,'' then every group is a
blend of shallower pedigrees. So, he says, ''I might be
closer in my DNA to an African than to another European in
the street.'' Genetics, he concludes, ''should be the last
nail in the coffin for racism.''

That's the utopian view. But there are still scientists who
claim that inferior genes plague certain races. J. Phillipe
Rushton, a professor of psychology at Canada's University of
Western Ontario, publishes books and articles claiming that
''Negroids'' have, on average, smaller brains, lower
intelligence, more ''aggressiveness,'' and less ''sexual
restraint'' than ''Caucasoids'' or ''Mongoloids.''

Rushton's views are on the extreme fringe, but even in
mainstream genetics, largely discredited concepts of race
persist. Scientific articles constantly speak of
''admixture'' between races, which implies a pure and static
standard for each race. ''Where did these standards come
from?'' asks Jackson. ''We've taken a 19th-century view of
racial variation and plugged in 20th-century technology.''
Indeed, the whole notion of racial standardsof a pure
Caucasian or a pure Negrois exactly what modern genetics
undermines. But, says Jackson, ''the philosophy hasn't
caught up with the technology.''

Over time, ''genetics will help beat down racist
arguments,'' says Eric Lander, a world-renowned geneticist
at M.I.T. ''But they will need to be beaten down, because
they will keep coming up.''